Field of the Invention
The present invention relates to a technique for measuring a three-dimensional shape of a measurement object.
Description of the Related Art
Three-dimensional shape measurement of an object surface using an image is used for various purposes, such as generation of a three-dimensional model based on a real thing, and measurement of a position and orientation of an object. In the techniques of three-dimensional shape measurement using an image, an active method in which pattern light referred to as structured light is irradiated is widely used because of its high reliability in measurement. As a device that performs irradiation of such pattern light, a projector is generally used which is a projection image display device. Typical examples of the three-dimensional shape measurement method using a projector include a space coding method and a phase shift method. The space coding method and the phase shift method are methods of performing three-dimensional shape measurement by projecting two-dimensional striped pattern light.
In the space coding method, light beams different in binary striped pattern are projected in time series to thereby stably associate coordinates on photographed image data with coordinates on image data projected from the projector. In the phase shift method, a plurality of striped pattern light beams which are shifted in phase of a sine-wave striped pattern are projected, and a change in density value of each pixel on the photographed image data is observed to thereby associate coordinates on photographed image data with coordinates on image data projected from the projector.
A camera and a projector used in the above-described method of projecting the striped pattern light are devices that input and output light through lenses. Therefore, distortion caused when light passes through a lens, referred to as distortion aberration, is added to image data photographed by the camera and image data output from the projector. To perform three-dimensional shape measurement with high accuracy, it is necessary to properly take into account an influence of such lens distortion. In correcting the lens distortion of the camera, it is a general practice that a distortion parameter is calibrated in advance, and two-dimensional coordinates on the photographed image data are corrected to coordinates without distortion in the three-dimensional shape measurement, using the distortion parameter. As to the lens distortion of the projector as well, by performing calibration in advance, it is also possible to perform the correction, based on a result of the calibration, similarly to the camera.
However, in the above-mentioned methods using the striped pattern light, such as the space coding method and the phase shift method, it is a general practice that striped pattern light substantially orthogonal to a direction of a base line connecting the camera and the projector is projected by the projector, for three-dimensional shape measurement. Therefore, to correct the lens distortion of the projector, although not necessary for the three-dimensional shape measurement, it is required to project pattern light orthogonal to the striped pattern light necessary for the three-dimensional shape measurement. On the other hand, R. J. Valkenburg and A. M. McIvor, “Accurate 3D measurement using a structured light system,” Image and Vision Computing, vol. 16, no. 2, pp. 99-110, 1998 discloses a method of correcting lens distortion of the projector without projecting unnecessary pattern light, by optimizing provisional three-dimensional coordinates calculated as the coordinates that is calculated ignoring lens distortion such that they satisfy a projection model including lens distortion.
However, projection of the pattern light orthogonal to the striped pattern light required for the three-dimensional shape measurement so as to correct the lens distortion of the projector naturally increases the number of pattern light beams to be projected. This causes a problem that a time required for photographing image data and processing image data is increased. On the other hand, in the method disclosed in R. J. Valkenburg and A. M. McIvor, “Accurate 3D measurement using a structured light system,” Image and Vision Computing, vol. 16, no. 2, pp. 99-110, 1998, although the measurement time is not increased due to projection and photographing of pattern light separately performed, it is required to carry out processing, such as calculating provisional three-dimensional coordinates, and performing non-linear optimization of the three-dimensional coordinates. These processing operations are required to be performed with respect to a lot of points on the image data, which brings about a problem that calculation is required to be executed an enormous number of times and hence it takes a very long time period to complete the calculation.